[dpdk-stable] patch 'mem: mark pages as not accessed when reserving VA' has been queued to stable release 19.11.3

[David Marchand]

Hello Luca,

On Fri, Jun 12, 2020 at 10:00 AM Luca Boccassi <bluca at debian.org> wrote:
> On Tue, 2020-06-09 at 15:14 +0100, Luca Boccassi wrote:
> > On Tue, 2020-06-09 at 14:45 +0100, Kevin Traynor wrote:
> > > Hi, discussed this patch for 18.11 with David and he pointed out that
> > > there is a fix for it on master:
> >
> > Uh, I wonder how it got left behind, strange - how critical is it? And
> > how safe is it to pick up now, when validation has already started?
>

- On the reason of why it was left behind, iiuc, Kevin thought it was
because of how the git-log-fixes.sh script works and this patch was
not marked as a fix in master.

- This first patch fixed the way we mark anonymous pages used by the
dpdk memory allocator.

At initialisation, the dpdk memory allocator reserves anonymous
mappings for later uses.
Before this patch, it was instructing the kernel the mapping was used
for reading with PROT_READ (which has some side effects, for
applications calling mlockall()).
The dpdk memory allocator has no actual need at the time of the
reservation, it switches to the right memory protection when using the
mapping.
So the first patch switched the memory protection to PROT_NONE when reserving.

Later when the dpdk memory allocator frees some memory and does not
use a mapping, the memory protection on this mapping is left untouched
as I forgot to set it back to PROT_NONE: this is what the 2nd patch
from Li Feng addresses.


The first patch landed in master at the start of 20.05 and we lived in
this state for a month.
The second patch got merged just before tagging 20.05-rc1.
"Real" validation happened with both patches applied.

I can understand your reluctance at touching the memory allocator now
that validation for 19.11.3 started, but with only the first patch, we
end up with an intermediate behavior in the dpdk memory allocator.
I would go with backporting the second patch.


-- 
David Marchand